(1) Field of the Invention
The present invention relates to an acoustic ranging system for multi-line towed acoustic arrays. More particularly, the present invention provides an acoustic ranging system that measures the magnitude of the separation of a pair of towed acoustic line arrays at a discrete point of the array in such a manner as to eliminate a multitude of measuring errors.
(2) Description of the Prior Art
Towed underwater acoustic sonar arrays are employed onboard surface ships, submarines and unmanned undersea vehicles to detect ships, marine life, marine geology, and other underwater sound sources. The towed sonar array comprises a long cable that trails the employed vehicle when the array is deployed.
Acoustic sensing elements, called hydrophones, are placed throughout the cable. The hydrophones of the array can be used individually to detect sound sources, but the real value of the hydrophones servicing the array is in the signal processing technique of sonar beamforming.
Sonar beamforming is a signal processing technique used in acoustic arrays for directional signal reception. The beamforming technique involves combining delayed signals from each hydrophone of the acoustic array at slightly different times, so that every signal reaches the output of the array at exactly the same time, making one loud signal, as if the signal came from a single sensitive hydrophone. By properly selecting the delayed signals, the array under consideration can effectively be steered to enhance the gain in one direction while decreasing gain in other directions. However, the relative position of each individual sensing element, such as the hydrophone, should be precisely known in order to properly select each time delay.
In a towed acoustic array that is lying perfectly straight, the positions are fairly straightforward to measure. However, various hydrodynamic forces acting on a towed array as the array travels through the water induce enough movement in the individual sensing elements that a straight-line approximation is no longer valid. Some techniques should be used to estimate the actual positions of the sensing elements and known so-called shape estimators perform this function.
Typical towed arrays shape estimators perform an integration operation over a combination of parameters, such as pressure (depth) and heading sensors positions located throughout the array, along with tow ship and array physical parameters so as to calculate the positions of the sensing elements. Current techniques are able to provide acceptable error margins for the determination of these positions in most applications for towed arrays.
In addition to the consideration of the difficulties of the estimation of the actual position of the sensing elements, one of the problems with towed arrays is their so-called left/right ambiguity. Without requesting the tow ship to perform maneuvers, it is difficult if not impossible to know if a sound source is coming from the left or right side of the array being utilized. More particularly, the beams that are generated by beamforming for an approximately linear array are conical in nature, leading to an ambiguity that rotates a full 360 degrees around the array.
To combat the ambiguity problem, some modern towed systems employ two or more arrays that are towed alongside each other. In this case, proper beamforming can estimate the relative depth/elevation and unambiguous direction of the sound source. In addition, utilization of the two arrays makes the whole system able to identify a quiet source on one side of the array in the presence of a loud source on the other side of the array. The difficulty with this technique is that the shape of the array estimation becomes more critical.
Errors that could previously be tolerated in a single array may no longer be acceptable in a multiline system that employs multiple arrays. This non-tolerance is more fully described by authors Cox, H., Lai, H., Heaney, K., & Murray, J. (2003) in the technical article entitled “Hybrid Adaptive Beamforming for Multi-line Arrays” discussed in Signals, Systems and Computers, (2003), and included in the Conference Record of the Thirty-Seventh Asilomar Conference on pages 1858-1862. As an example for this non-tolerance, a two degree measurement error in heading may not be significant in a single-line system, but in a multi-line system (if the two degree measurement error occurs) it may cause arrays to cross over each other which can be highly detrimental to the calculations being performed for the multi-line system.
Accuracy requirements for a multi-line towed acoustic array may be achieved by adding in a system for measuring line-to-line separation at discrete points along the arrays. Current systems do this acoustically, using one array in one line as the transmitter and the other array in the other line as the receiver. Raw data is sent from the measurement station in each line of the array back to the tow vehicle, where the two data sources are compared by means of an envelope correlator contained in the tow vehicle and used with the speed of sound in water to create a range estimate.
Measurement resolution is based on integration time and signal center frequency. As either of these two parameters increases, resolution improves. Integration time is fixed due to the motion of the array in the water. As the signal center frequency increases, the required data bandwidth servicing a hydrophone increases as well. To make useful measurements, a single range measurement system in a modern multi-line array may require sixteen or more times the bandwidth of a single hydrophone channel.
Since a number of stations are needed, this required bandwidth puts an extreme load on the array data telemetry system. Depending on the particular array, range measurement may require tens of kilo-samples per second using conventional techniques. It is therefore desired that a ranging system be provided that uses the equivalent of only a few samples per second.
To avoid the problems associated with range measurement systems having high bandwidths; a lower-bandwidth alternative to sending raw data can be achieved if the measurement stations in all arrays are synchronized in time. In this case, a unidirectional acoustic signal path is used. One array is the transmitter in one line and another is the receiver in the other line.
Since the receiver detects when the signal was transmitted because of the synchronization; the receiver is able to calculate transmit time internally and send only the correct result by way of the array telemetry to the tow ship. This solution is not viable in many towed array systems for the reason that synchronization among the engineering sensors (heading, depth, range) is not always guaranteed or achievable. It is desired that a ranging system be provided for multi-line towed acoustic arrays that does not require synchronization between its arrays, while still achieving accurate measurements.
A further additional consideration for ranging systems for multi-line towed acoustic arrays, is the ability for the receivers in the source array of the towed arrays to be able to distinguish the arriving signals from; 1) the transmitted signal emanating from the same hose of the multi-array having provisions for carrying both arriving and transmitting signals; and 2) the echo from different arrays when there is more than one repeater array present in the system. It is desired that a ranging system be provided for multi-line towed arrays that correctly interprets arriving and transmitted signals carried by the same hose and also correctly interprets echo signals from repeaters in the array.
Another parameter of interest is the parameter of dissimilar stretches in the towed arrays. Towed arrays stretch when under tension, and imperfect manufacturing tolerances may cause two towed arrays to stretch unevenly. This error, called array skew, increases toward the rear of the array being utilized. The amount of array skew may be calculated from the data that is collected for measuring separation of the elements of the array being utilized. It is desired that a ranging system be provided for multi-line towed acoustic arrays that accommodates for array skew in its measurement technique.